Both n-type chalcogenide compositions and/or p-type chalcogenide compositions have been incorporated into components of heterojunction photovoltaic devices. The p-type chalcogenide compositions have been used as the photovoltaic absorber region in these devices. Illustrative p-type, photovoltaically active chalcogenide compositions often include sulfides and/or selenides of at least one or more of aluminum (Al), copper (Cu), indium (In), and/or gallium (Ga). More typically at least two or even all three of Cu, In, and Ga are present. Such materials are referred to as CIS, CIAS, CISS, CIGS, and/or CIGSS compositions, or the like (collectively CIGS compositions hereinafter).
Absorbers based upon CIGS compositions offer several advantages. As one, these compositions have a very high cross-section for absorbing incident light. This means that CIGS-based absorber layers that are very thin can capture a very high percentage of incident light. For example, in many devices, CIGS-based absorber layers have a thickness in the range of from about 2 μm to about 3 μm. These thin layers allow devices incorporating these layers to be flexible. This is in contrast to silicon-based absorbers. Silicon-based absorbers have a lower cross-section for light capture and generally must be much thicker to capture the same amount of incident light. Silicon-based absorbers tend to be rigid, not flexible.
The n-type chalcogenide compositions, particularly those incorporating at least cadmium, have been used in photovoltaic devices as buffer layers. These materials generally have a band gap that is useful to help form a p-n junction proximal to the interface between the n-type and p-type materials. Like p-type materials, n-type chalcogenide layers can be thin enough to be used in flexible photovoltaic devices.
These chalcogenide based photovoltaic cells frequently also include other layers such as transparent conductive layers and window layers.
Heterojunction photovoltaic cells, especially those based on p-type and n-type chalcogenides, are water sensitive and can unduly degrade in the presence of too much water. Also, the thinner, flexible layers are vulnerable to thermal and other delamination or cracking stresses. Delamination and cracking not only can undermine device performance, but the resultant delamination and cracking also can exacerbate moisture intrusion. Therefore, to enhance service life, strong adhesion between device components is important to resist delamination, cracking, and moisture intrusion.
To protect heterojunction photovoltaic solar cells, especially chalcogenide-based solar cells, from detrimental moisture degradation, one or more hermetic barrier films can be deposited over the devices. FIG. 1 schematically illustrates one such approach. This device comprises a support 2, a backside electrical contact 3, an absorber layer 4, a buffer layer 5, a transparent conductive layer 6, an electrically conductive grid 7 and a barrier layer 8.
The barrier layer 8 of such a device may tend to show poor adhesion to the top surface(s) of the device. In particular, the adhesion between barrier materials and underlying transparent conductive materials and/or conductive collection grids may not be as strong as desired. Additionally, the adhesion between the grids and other materials, such as the TCO compositions, also may be poor. These issues can result in undue delamination 100 or in a rupture of the continuous hermetic barrier film and/or open pathways 110 allowing water intrusion to reach the chalcogenide compositions too easily. This can lead to subsequent device performance degradation and ultimately failure.
It is known to use silicon nitride films for passivation in the context of silicon-based solar cells. However, silicon-based solar cells tend to be thicker and much more rigid than chalcogenide-based cells. Accordingly, interlayer adhesion is much less of an issue in the context of silicon-based solar cells. Additionally, silicon-based solar cells have good moisture resistance so that moisture intrusion is much less of a concern for silicon-based solar cells.